Novel titanium refining process
Due to the extreme conditions used in this process (hot hydrogen fluoride) and several gas-phase steps requiring a complex apparatus, I don't expect
to try this at home, but it's a puzzle I've thought about for several years.
Titanium refining has some problems:
- direct electrolytic reduction of titanium oxide usually gives metal with some dissolved oxygen, which tends to be brittle and basically useless (the
FFC Cambridge process has yet to produce industrial titanium for this reason)
- titanium (IV) chloride is a covalent, volatile, toxic compound
So the proposed process has two steps:
- conversion of titanium ores to titanium (III) fluoride by precipitation of NH4TiF4 and its pyrolysis (basically known reactions)
- electrolytic reduction of TiF3 with a sacrificial tungsten anode (new)
The reduction of TiO2 to Ti2O3 with hydrogen (inter alia, [1]) or by carbothermal methods [2] has been documented many times. Unfortunately, it's hard
to find good examples because they're all complaining about the failure to produce Ti metal directly this way. Alternatively, waste titanium can be
used to reduce TiO2, itself being hard to use directly due to dissolved oxygen.
But owing to the solubility of titania in fluoride solution, the direct leaching of titanium ores with hydrogen fluoride has been proposed as well
[3]. The direct reduction of the solution with hydrogen is not described, but it is energetically favorable if we suppose that the relevant ionic
species are TiO(2+), H2/H+ and Ti(3+). Usually titania is solubilized using ammonium fluoride or bifluoride, due to the difficulty of working with
hydrofluoric acid.
Once titanium is in the form of titanium (III), there is a known process for producing anhydrous NH4TiF4 by precipitation from hydrofluoric acid [4].
This is the key step, because oxygen and iron impurities significantly degrade the properties of the product, and they are removed in this step.
NH4TiF4 decomposes under heating to give TiF3.
Once TiF3 is produced, it can be dissolved in molten fluoride salts and electrolysed. According to the standard electrode potentials, the oxidation
Ti3+ >> Ti4+ requires more energy per electron than the oxidation of W >> W6+. So at a tungsten anode, WF6 is produced and under these
conditions is extremely volatile. While not a particularly friendly compound, WF6 is still preferable to fluorine gas, which will corrode any anode
and is generally avoided whenever possible. The cathode is probably a piece of titanium wire, which accumulates oxygen-free titanium from the melt.
WF6 in turn is used (today) primarily for tungsten plating by the decomposition of a mixture of WF6 and H2 gases onto a hot surface, a very old
process [5], which is proposed to regenerate the anode.
One possibility is that the anode could comprise most of the reaction vessel for electrolysis, which would reduce the needed rate of deposition. If
the tank has a surface area of one square meter, several pounds of tungsten are deposited per mm of thickness. But this might not be practical.
Alternatively, the anode is an inert wire, which is coated in tungsten, then used for electrolysis, and then recycled by tungsten vapor deposition.
So effectively, all of the intermediates are recycled and the net reaction is just TiO2 + 2 H2 >> Ti + 2 H2O. Oxygen is rigorously excluded from
the product.
The downside is the use of lots of hot corrosive gas. But the competition is all based on TiCl4, using a carbothermal reduction in the presence of
chlorine. So we're not much worse. But I wonder if Sciencemadness could suggest some improvements.
1: https://link.springer.com/article/10.1023/A:1015378931111
2: https://www.sciencedirect.com/science/article/pii/S027288421...
3: https://www.scientific.net/SSP.265.542
4: https://www.jstage.jst.go.jp/article/nikkashi1972/1973/1/197...
5: https://iopscience.iop.org/article/10.1149/1.2426649/meta
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